It develops from the nasal (olfactory) placode, at the anterior edge of the neural plate (cranial nerve zero). It is a chemoreceptor organ which is completely separated from the nasal cavity the majority of the time, being enclosed in a separate bony or cartilaginous capsule which opens into the base of the nasal cavity. It is a tubular crescent shape and split into two pairs, separated by the nasal septum. It is the first processing stage of the accessory olfactory system, after which chemical stimuli go to the accessory olfactory bulb, then to targets in the amygdala and hypothalamus. The VNO has two separate types of neuronal receptors, V1R and V2R, which are seven-transmembrane receptors that are coupled to guanosine triphosphate-binding proteins. The receptors are distinct from each other and from the large family of receptors in the main olfactory system. Evidence shows that the VNO responds to nonvolatile cues which stimulate the receptor neurons. Its presence in many animals has been widely studied and the importance of the vomeronasal system to the role of reproduction and social networking in animals has been shown in many studies. Its presence and functionality in humans is widely controversial, though most studies agree the organ regresses during fetal development.
The VNO is a tubular crescent shape and split into two pairs, separated by the nasal septum. The crescent lumen is lined with receptor neurons on the medial concave side and is filled with fluid from the VN glands. There VN neurons are isolated from the nasal cavity and therefore isolated from the air stream that passes during normal respiration. This means that a stimulus requires arousal of the vascular pump which is lateral to the lumen. The medial, concave area of the lumen is lined with a pseudo stratified epithelium that has three main cell types: receptor cells, supporting cells, and basal cells. The supporting cells are located superficially on the membrane while the basal cells are found on the basement membrane near the non sensory epithelium. The vomeronasal sensory cells form in the olfactory placode along with other sensory olfaction neurons. The vomeronasal sensory neurons communicate with the hypothalamus to change neuroendocrine function. These sensory receptors are often referred to as pheromone receptors since vomeronasal receptors have been tied to detecting pheromones.
The receptor cells are G-protein-coupled receptors which detect the pheromones, which are frequently referred to as pheromone receptors. The receptor neurons possess apical microvilli whose axons merge together to form VN nerves which move from the paired olfactory bulbs to the main olfactory bulb, entering the posterior dorsal aspect through the AOB. There have been two different G-protein-coupled receptors identified in the VNO, each found in distinct regions. These are V1 and V2. V1 and V2 are seven transmembrane receptors which are not closely related to the main olfactory receptors.
The vomeronasal organ’s sensory neurons act on a different signaling pathway than that of the main olfactory system’s sensory neurons. Upon stimulation activated by pheromones, IP3 production has been shown to increase in VNO membranes in many animals, while adenylyl cyclase and cyclic adenosine monophosphate (cAMP), the major signaling transduction molecules of the main olfactory system, remain unaltered. This trend has been shown in many animals, such as the hamster, the pig, the rat, and the garter snake upon introduction of vaginal or seminal secretions into the environment.
V1Rs and V2Rs are suggested to be activated by distinct ligands or pheromones. The evidence that Gi and Go proteins are activated upon stimulation via different pheromones supports this.
In some other mammals, the entire organ contracts or pumps in order to draw in the scents.
Some mammals, particularly felids and ungulates, use a distinctive facial movement called the flehmen response to direct inhaled compounds to this organ. The animal will lift its head after finding the odorant, wrinkle its nose while lifting its lips, and cease to breathe momentarily. Flehmen behavior is associated with “anatomical specialization”, and animals that present flehmen behavior have incisive papilla and ducts, which connect the oral cavity to the VNO, that are found behind their teeth.
Another study conducted by Beauchamp et al. investigated the role of the VNO in male guinea pigs social behavior. Half of the guinea pigs vomeronasal systems were removed, while the other half were put under fake surgeries with their vomeronasal systems left intact. The findings suggested that the VNO in the male domestic guinea pig is necessary for the maintenance of normal responsiveness to sex odors. However, “in its absence, other sensory systems are capable of maintaining normal sexual behavior under conditions of laboratory testing.”
These behavioral studies show the importance of the vomeronasal system in animals’ social networks and everyday activities. The importance of the vomeronasal system to the role of reproduction and social networking has been shown in many studies.
Many studies have been performed to find if there is an actual presence of a VNO in adult humans. Trotier et al. estimated that around 92% of their subjects that had no septal surgery had at least one intact VNO. Kajer and Hansen, on the other hand, stated that VNO structure disappeared at later stages in development. Won (2000) found evidence of a VNO in 13 of his 22 cadavers (59.1%) and in 22 of his 78 living patients (28.2%). Given these findings, some scientists have argued that there is a VNO in adult humans. However there is no reported evidence that humans have active sensory neurons like those in working vomeronasal systems of other animals. Furthermore, there is no evidence to date that suggests there are nerve and axon connections between any existing sensory receptor cells that may be in the adult human VNO and the brain. Likewise, there is no evidence for any accessory olfactory bulb in the adult humans, and the key genes involved in VNO function in other mammals have pseudogeneized in humans. Therefore while the presence of a structure in adult humans is debated, a review of the scientific literature by Tristram Wyatt concluded, "most in the field... are sceptical about the likelihood of a functional VNO in adult humans on current evidence."
Behavioral changes of patients after orthognathic surgery develop on the basis of the loss of vomeronasal organ: a hypothesis.(Hypothesis)(Report)
Jan 22, 2009; Authors: René Foltán ; Ji?í ?edý (corresponding author) BackgroundOrthognathic surgery is a surgical discipline aimed at...